This priority letter Airworthiness Directive (AD) is prompted by reports of the
potential for carbon monoxide gas to leak into the cabin heating system and the cabin of
certain Cessna Model 172R airplanes. This condition, if not corrected, could result in
passenger and pilot injury with consequent loss of control of the airplane.

Cessna Aircraft Company has reported that a quality control problem exists with
Aeroquip engine exhaust mufflers installed on certain Cessna Model 172R airplanes. Cessna
recently notified the FAA that the Aeroquip muffler, part number (P/N) 00624-NH4000011-10
71379 0554011-2, is installed in approximately 250 Cessna Model 172R airplanes. Cessna has
already determined through pressure testing that approximately 5 out of the 25 tested
mufflers manufactured by Aeroquip are leaking. These inadequate or failed weldments will
permit exhaust gas (including carbon monoxide) leakage from the muffler, and consequently
into the airplane's cabin and cock-pit area.

After examining the circumstances and reviewing all information related to the
situation described above, the FAA has determined that AD action should be taken on Cessna
Model 172R airplanes to prevent carbon monox-ide gas from entering the airplane's cabin
heating system and cabin, which, if not corrected, could result in passenger and pilot
injury with consequent loss of control of the airplane. Since an unsafe condition has been
identified (carbon monoxide leakage into the cabin area) that is likely to exist or
develop on other Cessna Model 172R airplanes of this same type design, this AD requires,
prior to further flight (1) de-activating the cabin heating system, and (2) fabricating
and installing a placard within the pilot's clear view, using 1/8-inch letters with the
following words: "CABIN HEATER INOPERATIVE."

This AD also requires, within the next 50 hours time-in-service (TIS), replacing the
Aeroquip engine exhaust muffler (P/N 00624-NH4000011-10 71379 0554011-2). If replacement
parts are not available, the airplane may continue operation with the heating system
disabled for a period not to exceed 6 calendar months after the effective date of this AD.

This rule is issued under 49 U.S.C. Section 44701 (formerly section 601 of the Federal
Aviation Act of 1958), pursuant to the authority delegated to me by the Administrator, and
is effective immediately upon receipt of this priority letter.

NOTE 1: The letters "PT" or "PTT" stamped in the right-hand
external ring that supports the muffler cabin heater shroud indicate that Cessna has built
or re-built the part. Parts marked in this manner are not Aeroquip parts.

NOTE 2: This AD applies to each airplane identified in the preceding
applicability provision, regardless of whether it has been modified, altered, or repaired
in the area subject to the requirements of this AD. For airplanes that have been modified,
altered, or repaired so that the performance of the requirements of this AD is affected,
the owner/operator must request approval for an alternative method of compliance in
accordance with paragraph (h) of this AD. The request should include an assessment of the
effect of the modification, alteration, or repair on the unsafe condition addressed by
this AD; and, if the unsafe condition has not been eliminated, the request should include
specific proposed actions to address it.

Compliance: Required as indicated in the body of this priority letter AD, unless
already accomplished. To prevent carbon monoxide gas from entering the airplanes
cabin heating system and cabin, which, if not corrected, could result in passenger and
pilot injury with consequent loss of control of the airplane, accomplish the following:

(a) Prior to further flight after receipt of this priority letter, AD, de-activate the
cabin heating system by ensuring that the valve mechanism is functional, and that the
cabin heat valve lever is safety wired in the down "off" position.

(b) Prior to further flight after receipt of this priority letter AD, fabricate and
install a placard near the cabin heat control knob, within the pilot's clear view, using
at least 1/8-inch letters with the following words: "CABIN HEATER INOPERATIVE"

(c) Within the next 50 hours time-in-service (TIS) after receipt of this priority
letter AD, replace the engine exhaust muffler with a muffler having one of the following
part numbers (P/N) in accordance with the appropriate Cessna maintenance manual:
00624-NH4000011-10 71379 0554011-2-PTT, or 0554011-2, or 0554011-6, or an FAA-approved
equivalent part number.

NOTE 3: P/N 0554011-2 will have "PT" stamped on the right-hand
external ring that supports the muffler; and P/N 0554011-6 may have "PT" stamped
on the right-hand external ring.

(d) If parts are not available for the replacement required in paragraph (c) of this
AD, the airplane may continue to be operated for a period not to exceed 6 calendar months
from the date of receipt of this priority letter AD, provided the cabin heating system
remains de-activated.

(e) The cabin heating system may be re-activated and the placard required in paragraph
(b) of this AD may be removed, once the muffler is replaced in accordance with this
priority letter AD.

(g) Special flight permits may be issued in accordance with sections 21.197 and 21.199
of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the airplane to
a location where the requirements of this AD can be accomplished, provided the airplane
cabin heater system is not used during that flight.

(h) An alternative method of compliance or adjustment of the compliance times that
provides an equiva-lent level of safety may be approved by the Manager, Wichita Aircraft
Certification Office, 1801 Airport Road, Rm. 100, Mid-Continent Airport, Wichita, Kansas
67209. The request shall be forwarded through an appropriate FAA Maintenance Inspector,
who may concur or comment and then send it to the Manager, Wichita Aircraft Certification
Office.

NOTE 4: Information concerning the existence of approved alternative methods of
compliance with this AD, if any, may be obtained from the Wichita Aircraft Certification
Office.

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